DESCRIPTION: The broad, long-term objective of this research program is to develop general technology to conditionally regulate protein function at the level of the protein molecules rather than by targeting the DNA or mRNA precursors that encode a protein-of-interest. This technology is highly specific for the targeted protein and provides rapid and tunable control of protein function using cell-permeable small molecules or non-toxic light. The goal is to engineer small protein domains called destabilizing domains that are rapidly degraded when expressed in mammalian cells. The instability of destabilizing domains is faithfully conferred to other proteins fused to these small domains, allowing researchers to predictably control the levels of any protein-of-interest. One specific aim of this research program will result in new destabilizing domains that are intrinsically fluorescent, allowing researchers to quantify protein levels using optical methods. A second aim of this research is to produce destabilizing domains that are regulated by blue light rather than small molecules, thus enabling the control of protein stability with high spatial resolution. A third aim of these studies will provide a family of destabilizing domains whose cellular levels can be raised by treatment with one ligand or whose cellular levels can be made to fall by treatment with a different ligand. In this case a single destabilizin domain could be used to test the effects of overexpression with one ligand or strong knock-down with a different ligand in an isogenic background. The fourth aim of this research program is to produce destabilizing domains for use in Apicomplexan parasites such as Plasmodium falciparum and Toxoplasma gondii. A mechanistic understanding of how these domains are recognized and degraded in mammalian cells will make this technology more useful to users. These studies may also reveal general mechanisms that cells use to recognize and degrade unfolded or misfolded proteins, and these mechanisms are likely relevant to human diseases.